After reading "the audiophile's project sourcebook" by G. Randy Sloan, I am intrigued by phase linear filters. I have even found a DIY kit on ebay for them.

Has anyone ever built one or used one? Are they any good or better for a bi-amp setup than an electronic crossover? and if so, why don't you see more of them used. Just wanting some real world feedback before spending time and money to build one.

I designed and built one years ago as a sub-woofer crossover. I didn't call it 'phase linear' though - rather a subtractive XO. The general principle is using all-pass filters to delay compensate for low pass functions so that high pass can be obtained by simple subtraction.

Definitely worth playing with. The downside is the use of more opamp stages, so it all depends how transparent you find opamps....

Oh and there's something else - the attenuation at the LF end of the HP function rather depends on gain matching, because its the subtraction of two near-identical signals. Works brilliantly in simulation but with real hardware you do have to pay attention to gain stability vs time and temp.

__________________When irony first makes itself known in a young man's life, it can be like his first experience of getting drunk - Robertson Davies

I know what it says, but that's not a phase-linear crossover in the Slone book. As noted by abraxalito, it's a subtracting crossover that uses an all-pass network in conjunction with the low-pass network and to create the high-pass output.

Only crossovers that exhibit a flat (linear) phase response with the outputs summed can be considered linear-phase. (That one doesn't qualify.)

I my experience of building active crossovers with opamps (over many years) the layout is the primary determinant of how it sounds. Beyond that, simpler is generally better. So the answer to question 1 is 'It depends how you implement it'. I have no experience at all of ebay pcbs so can't even hazard a guess to answer 2).

As to the last question - go for whatever topology seems more interesting. The more involved you get with a circuit, the more you 'own' it so to speak, the more satisfaction you get from it. And that satisfaction alone will make it sound better to you.

__________________When irony first makes itself known in a young man's life, it can be like his first experience of getting drunk - Robertson Davies

All other things being equal, a subtractive crossover (as exampled by Slone) should not "sound" any different than any other topology. The phase response of that crossover can be duplicated by other types, so there isn't any inherent advantage in that aspect.

Some folks might prefer that design (subjectively) because it doesn't require/utilize series-connected capacitors. Maybe that's something to be worried about, but maybe not. When subjective evaluations are the primary determinite then the opinions can be all over the place.

Anyways, maybe your original question is more regarding the audibility of non-flat phase response. There are varying opinions on whether it is even audible and how best to evaluate it. Generally, speaker designers claiming audible superiority of linear-phase response are not making an apples/oranges comparison since other variables are involved.

Thanks for all of the good input. For now I think I will focus my money on upgrading other components in the system and stick with a standard electronic crossover. After everything else is optimized, then maybe I will go back and build the design from his book and see if I can hear any difference.